Blogs

 

Case study: Saving a million dollar bridge

By intouch * posted 18-10-2018 08:06

  

Relief – that was David Edge’s predominant emotion when he heard the Leslie Hills Bridge was salvageable.


LH_bridge_2.jpgIt certainly didn’t look like it could be saved. It was 12 September 2013, and the 72m-long, 3.66m-wide, three-span continuous steel structure had buckled under the weight of a collapsed pier that had been dislodged by fast-moving floodwaters.

Edge, Hurunui District Council’s Manager Infrastructure Services, was staring down the barrel of replacing a $1 million asset that provided essential access for the community, the local dairy industry and tourists. 

“It was pretty horrific. I would have thought the bridge was in dire straits, given the fact that the damaged pier was an extra piece of weight that was suspending and pulling the bridge down,” Edge says.

Built in 1969, the foundations of the bridge’s northern pier had been encased in a shallow socket in brittle rock, with no rock anchors.

“When they built the bridge back in 1969 it was a decision they took or it was maybe best practice that they applied at that stage. In the three-yearly bridge inspections conducted by WSP-Opus and the poor as-built data prevailing, it wasn’t something that came up on their radar until it physically failed,” Edge explains.

“The only recorded condition issues were minor cracking and spalling of concrete, leaking of the expansion joints and minor corrosion of the beams.”  

Nevertheless, when the Waiau River rose to 1 in 2-year levels, the pier failed and the continuous steel beams at the northern end abutment rotated and three out of four bolts pulled free of the grouted ducts.

Assessing the options

LH_bridge_4.pngWSP-Opus inspected the bridge, and delivered the good news: the bridge could be rehabilitated. The council and the New Zealand Transport Agency (who co-invested in the infrastructure) were then presented with their options, which ranged from closing the bridge to demolishing it and building a new one.

They chose ‘option 3’, which Edge says ticked both the level-of-service and affordability boxes.

Option 3 involved releasing connections at the north abutment to allow further rotation, cutting the concrete deck at the south pier to allow for rotation and yielding of beams, installing extra cross bracing frames to prevent lateral torsion buckling, over ‘jack’ the frame 200mm off the temporary pier to release the deck onto the permanent newly constructed pier, drilling 600mm diameter holes into solid rock to accommodate a new pier foundation, repairing kerbs/deck/handrails and creating new connections at the north abutment.  

“The original scope was to get back at least the original whole of life left in the bridge. This business case assessment provided us the highest level of benefit that was defined in terms of what we originally wanted, and it mitigated the majority of the risks,” Edge says.

“It’s fit for purpose being a low traffic volume bridge, but it’s quite an important connector link to be considering anything like a lessor bridge.”

As a small rating district council located in North Canterbury, the cost of building a replacement bridge would have had flow-on effects for the rest of the infrastructure works program, even with NZTA contributing 51%.

“I was looking at major dollars here, because if you’re looking at a structure that’s going to cost $1 million to replace, we’re $490,000 in the hole just for one structure. In our district $60,000 is a 1% rates increase,” Edge says.

“We’ve got so many other parts of my infrastructure that are ageing, we’ve got legislative compliance requirements, so to have invested this amount of money into a single bridge asset where there are 286 bridges in my district, it would have put us right on the financial back foot.

“To know we were in a position to get a bridge back in place and we could meet all the community’s requirements was a massive relief. It was the fact that we could get some life out of an original investment too, because when you dismantle something, you’re throwing away all that investment that you originally put into the whole-of-life, so that’s a discount factor that need’s to be taken into account as well when you look at those options and price comparisons.”

Reopened in eight days

LH_bridge_3.jpgEdge, who presented the Leslie Hills Bridge remediation case study at a Sydney bridge conference last month, says the Council’s priority was to reopen the bridge to light vehicles as soon as possible.

Although rated as a low-traffic area with average daily traffic of 114 vehicles (18% of those heavy) the bridge provided the local community with access to and from State Highway 7, was trafficked by Fonterra milk trucks, the school bus and international tourists accessing the renoun ‘Alpine Pacific Triangle’ drive. 

He feared it would take as long as four weeks to reopen. Remarkably, the team of council staff, consultants and contractors were able to put a temporary pier in place and reopen the bridge to light vehicles within eight days – a phenomenal feat that Edge attributes to 24/7 efforts, excellent stakeholder communication and a sprinkling of good luck.
 
The full remediation works were completed outside the desired nine months, delayed due to the bulk of consultants and contractors being drawn into the post-Christchurch earthquake repair effort. The final cost of remediation, including for design, variation and consent costs, was just in excess of $500k, about half the cost of what it would have been to replace the bridge (including design, variations and consent costs).

“It was a brilliant game plan that came together. Everyone played their part, and each part fitted in perfectly, otherwise we would have never made those timeframes,” Edge says.

“It’s an exceptionally short time for recovery and was an absolutely amazing project with many areas of innovation applied successfully – that’s why I think it’s been held in such high esteem and has been put forward as a project of renown.”

The project was recognised in 2017 with an IPWEA NZ Excellence Award, winning the Excellence in Maximising Asset Performance category, and is a finalist in the New Zealand Bridges Awards. 

Resilient against earthquakes 

LH_bridge_final.jpgOne if the most successful aspects of the rehabilitated bridge was an improved resilience to earthquakes.

“When you think back to the design criteria in 1969, they certainly wouldn’t have been putting any notable seismic resistance into their designs back then, and with a new standard to meet the minimum one in 1000 year event, I wanted that standard to be applied to this bridge,” Edge says.

“We’ve got the Alpine Fault (AF8), which is an fault line that eventually will go. It goes every 300 years, and we’re about 350 years since the last one. We’ve looked across all our bridging infrastructure to start putting some resilience in place. Connectivity is going to be key in terms of lifelines when this event eventually happens, and bridges are a key part of that.”

The bridge’s seismic resistance was put to the test in November 2016, when a 7.8 earthquake event was registered 20km away, with in excess of 3.3 vertical g-forces.

“You can only imagine what the bridge experienced, and to have met the outcomes which were well in excess of 1 in 1000 year is an amazing design and construction accomplishment. Subsequently, we’ve had some flood issues come through there again in excess of the one that caused the original problem, and it’s resisted with flying colours,” Edge says proudly.

Response project timeline

12 September 2013 (afternoon) – The bridge pier collapses. Initial visual and survey assessment is undertaken and the bridge is safely cordoned off. A community meeting notification is sent out.

13 September – The bridge was surveyed, noted continued deflection with partial pier support, and decision to jack bridge for temporary pier and release northern pier. A community meeting is held on site.

14 September – A crane is used to release northern pier and the bridge ‘springs’ back.

15 September – WSP-Opus undertake an inspection and resurvey. The deck joints are closed marginally and 110mm vertical return at northern pier location. A temporary pier is designed and sent for manufacture.

17 September – Smith Crane and Construction ‘jack’ the bridge 350mm with a crane, place in the temporary, manufactured pier and drill rock anchors. Proof loaded deck 4.5T with 10mm deflection North Pier and 7mm South Pier.

19 September – More proof loading was done, with no significant deflection noted.

20 September – Bridge safely opened to light vehicles (4000kg max gross mass and max 20 kph). The bridge is surveyed every three days (two weeks), then every week (two months), then monthly.
0 comments
31 views